The present invention relates to a distance measuring system for measuring a distance to an object to be measured by projecting a distance measuring light beam toward the object and by receiving the reflection light beam from the object, and in particular, to a distance measuring system for measuring a distance by receiving the light beam reflected from a surface of an object to be measured or by installing a prism on the object to be measured and by receiving the reflection light beam reflected from the prism.
In recent years, a new type of a distance measuring system has appeared, by which a distance to an object to be measured can be determined without using a prism for reflecting a distance measuring light beam. This new type of the distance measuring system measures the distance by receiving the weak reflection light beam reflected from the surface of the object to be measured itself or by emitting a pulsed light beam with high peak power and by receiving the reflection light beam reflected from the surface of the object to be measured itself.
Also, a type of a distance measuring system has appeared recently, which can measure a distance in a prism measurement mode using a prism and also in a non-prism measurement mode without using a prism. In this measuring system, both the prism measurement mode and the non-prism measurement mode can be selected depending on each application or situation. Because the measurement mode can be selected, there is no need to provide a light wave range-finder in the prism measurement mode and a laser range-finder in the non-prism measurement mode, and this is very convenient and advantageous for the user. When the measuring position should be clearly defined and the measurement should be performed with high accuracy in the distance measuring system, which has functions to measure the distance in both of the two measurement modes, i.e. the prism measurement mode and the non-prism measurement mode, the distance is measured using the prism. If it is difficult to install the prism on the object to be measured, and if the measurement may not be so accurate as in the measurement using the prism and the approximate distance would be measured, the distance is measured without using the prism.
In a conventional type of a distance measuring system with a non-prism measuring function, the distance is measured by receiving the reflection light beam from the object to be measured with a very small reflection light amount. For this reason, it is designed in such manner that photodetection sensitivity is set to a higher value or an exit light amount of the distance measuring light is set to a higher value. In this respect, even when the prism is not correctly collimated in the prism measurement mode, the measurement may be performed using reflection light beam from the surface of an object other than the prism, e.g. a natural object. In this case, the incorrectly measured distance value may be displayed on a display unit of the distance measuring system.
In particular, when the user measures the distance using the prism, the distance must be measured with high accuracy as described above. If the distance is measured even though the prism is not correctly collimated, the user may acquire an incorrect distance value without becoming aware of incorrectness of the data. In addition, when there is an obstacle before the prism and a part of the distance measuring light beams hits the obstacle, the user may measure the distance to the obstacle without being aware that it is not the object to be measured. Furthermore, even when the user becomes aware, it is difficult to judge which is correctly measured, the obstacle or the object to be measured.
These problems did not occur in case of the conventional type of a light wave range-finder, which can measure the distance only in the prism measurement mode. That is, in case of the conventional type of the light wave range-finder, the distance cannot be measured unless the sufficient light amount is reflected as the reflection light beam from the prism. If the distance is not measured in the conventional type of the system, it means that the prism is not correctly collimated. Thus, incorrect measurement of an object other than the prism did not occur in the conventional type of the system.
It is an object of the present invention to provide a distance measuring system, by which a type of an object to be measured is judged from a photodetection amount of a light beam reflected from the object to be measured and from a measured distance value, and it is possible to eliminate incorrect measurement in case of prism measurement.
To attain the above object, the distance measuring system according to the present invention comprises a control arithmetic unit, a light emitting unit for emitting a measuring light beam and a photodetection unit for receiving a reflection light beam from an object to be measured, and the system being used for measuring a distance by receiving the reflection light beam from the object to be measured, wherein the control arithmetic unit compares a signal based on photodetection amount of the light from the object to be measured as well as a result of distance measurement with a reference data prestored in the control arithmetic unit relating to reflection of the object to be measured, and judges the object to be measured based on a result of the comparison. Also, the present invention provides the distance measuring system as described above, further comprising a density filter for adjusting the photodetection amount of the light beam from the object to be measured, wherein the signal based on the photodetection amount represents a density position of the density filter, and the reference data relating to reflection of the object to be measured is obtained by associating a measured distance with the density position of the density filter. Further, the present invention provides the distance measuring system as described above, wherein the density filter is a disk where density is continuously changed in a circumferential direction, the density filter is rotated by a stepping motor, and the density position corresponds to a number of rotating steps of the stepping motor. Also, the present invention provides the distance measuring system as described above, wherein the reference data relating to the reflection of the object to be measured contains change of the photodetection amount due to weather conditions as a tolerance value. Further, the present invention provides the distance measuring system as described above, further comprising a display unit, wherein a result of judgment on the object to be measured is displayed on the display unit. Also, the present invention provides the distance measuring system as described above, wherein there are provided at least a prism measurement mode and a non-prism measurement mode, and when the prism mode is selected, the distance is displayed on the display unit only when the object to be measured is judged as a corner cube, and the fact that the object to be measured is not a corner cube is displayed on the display unit when the object to be measured is not judged as a corner cube. Further, the present invention provides the distance measuring system as described above, wherein photodetection sensitivity can be automatically changed over according to the photodetection amount of the light beam from the object to be measured, the object to be measured is judged according to the photodetection amount, and a result of judgment on the object to be measured is displayed on the display unit.